How to check with Javascript if a Firefox 3 add-in / extension is installed

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Actually this question is based on a Blog Entry, which discusses the topic for FF2. But how does this work with FireFox 3?

I know that there must be a workaround, because recently I visited a site saying: "Please disable AdBlock+ before entering this site" :(


There's no general-purpose way to do that, for security reasons. Even the FF2 version mentioned there was specific to that particular extension -- you can't assume identically-named and -placed icons for all extensions.

The AdBlock-spotters, as Stephen presumed, tend to be behavior-based and specific to that extension. Likewise Greasemonkey-spotters, etc.